Most distributed systems are large-scale and dynamic. Communication has thus become a critical issue and the shared memory, which is a reliable paradigm of communication, allow distributed nodes to exchange information through read and write operations. In this article, I introduce the challenges related to emulating a shared memory in a dynamic message passing environment where nodes constantly leave the system and where new nodes arrive in an unpredictable manner. This article surveys the work on quorum systems and reconfigurable systems, and present solutions that cope with crash failures, dynamism and limited complexity.